1. Field of the Invention
The present invention relates to computer processing of data defining a three-dimensional (3D) computer model of an object.
2. Related Background Art
Many methods are known for generating a 3D computer model of an object.
The known methods include “shape-from-silhouette” methods, which generate a 3D computer model by processing images of an object recorded at known positions and orientations to back project the silhouette of the object in each image to give a respective endless cone containing the object and having its appex at the position of the focal point of the camera when the image was recorded. Each cone therefore constrains the volume of 3D space occupied by the object, and the intersection of the cones, which approximates the object and is known as the “visual hull” of the object, is calculated.
Examples of shape-from-silhouette methods are described, for example, in “Looking to build a model world: automatic construction of static object models using computer vision” by Illingworth and Hilton in Electronics and Communication Engineering Journal, June 1998, pages 103–113, and “Automatic reconstruction of 3D objects using a mobile camera” by Niem in Image and Vision Computing 17 (1999) pages 125–134. The methods described in both of these papers calculate the intersections of the silhouette cones to generate a “volume representation” of the object made up of a plurality of voxels (cuboids). More particularly, 3D space is divided into voxels, and the voxels are tested to determine which ones lie inside the volume defined by the intersection of the silhouette cones. Voxels inside the intersection volume are retained to define a volume of voxels representing the object. The volume representation is then converted to a surface model comprising a plurality of polygons for rendering.
“A Volumetric Intersection Algorithm for 3d-Reconstruction Using a Boundary-Representation” by Martin Löhlein at http://i31www.ira.uka.de/diplomarbeiten/da_martin_loehlein/Reconstruction.html discloses a shape-from-silhouette method of generating a 3D computer model which does not result in a voxel representation. Instead, the intersections of the silhouette cones from a plurality of images are calculated directly. More particularly, the method starts with a cube containing the object, and intersects it with the first silhouette cone to give a first approximation of the object. This approximation is then intersected with the next cone to give a second approximation, and so on for each respective silhouette cone. To intersect a silhouette cone with an approximation, the cone and the approximation are projected into the image from which the cone was taken. This reduces the cone to the 2d-polygon (silhouette) from which it was made and reduces the approximation from 3d-polygons to 2d-polygons. The cone polygon is then intersected with all the approximation's polygons using a conventional algorithm for 2d-polygon intersection.
The inventor of the present invention has realised that a visual hull computer model of an object often suffers from the problem that artefacts comprising concavities and convexities appear in the computer world which do not exist on the object in real-life.
In particular, the inventor has identified two types of convex and concave artefacts which decrease the accuracy of the 3D computer model of an object and which are a particular problem. These are convex artefacts on top of planar surfaces forming a “dome” on the planar surface, and convex and concave artefacts in high curvature surface regions forming “creases” and “folds” in the surface that are not present on the object.
The present invention has been made with these problems in mind.